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1 of 39 © Boardworks Ltd 2007
Kinetics
© Boardworks Ltd 20072 of 39
© Boardworks Ltd 20073 of 39
What does rate of reaction mean?
The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow.
What is the rate of these reactions?
The speed of a reaction is called the rate of the reaction.
rusting baking explosion
slow fast very fast
Airbag Reaction
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Rates of reaction
Why are some reactions faster than others?
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Measuring the rate of a reaction
The change in concentration can be measured by using any property that changes during the reaction. These may include
Mass or volume changes for gaseous reactions.
Change in pH for reactions involving acids and bases.
Changes in conductivity measurements for reactions involving electrolytes.
Use of a spectrometer or colorimeter for reactions involving change in colour.
© Boardworks Ltd 20076 of 39
Collision Theory
For a reaction to take place between two reactant particles, three conditions are necessary:
The two particles must collide with each other. They must collide in the correct orientation, so
that the reactive parts of each particle will come into contact with each other.
The reactants must collide with sufficient kinetic energy to bring about the raction.
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Graphing rates of reaction
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Reactant–product mix
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Activation Energy
The minimum amount of energy required to bring out the reaction is called the Activation Energy
The activation energy (Ea) for the forward reaction is shown by (a)
The activation energy (Ea) for the reverse reaction is shown by (b)
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Setting up rate experiments
What equipment is needed to investigate the rate of hydrogen production?
gas syringe
rubber bung
rubber connecterglass tube
conicalflask
magnesium
hydrochloricacid
© Boardworks Ltd 200711 of 39
hydr
ogen
pro
duce
d (c
m3)
time (seconds)10 20 30 40 50
10
20
30
40
50
60
70
00
x
y
Calculating rate of reaction from graphs
rate of reaction =
x
y
rate of reaction =
20 s
45 cm3 rate of reaction = 2.25 cm3/s
The gradient of the graph is equal to the initial rate of reaction at that time
How can the rate of reaction be calculated from a graph?
© Boardworks Ltd 200712 of 39
Slower and slower!
Reactions do not proceed at a steady rate. They start off at a certain speed, then get slower and slower until they stop.
As the reaction progresses, the concentration of reactants decreases.
This reduces the frequency of collisions between particles and so the reaction slows down.
percentage completion of reaction
100%0% 25% 50% 75%
reactantsproduct
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The reactant/product mix
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Changing the rate of reactions
increased temperature
increased concentration of dissolved reactants, and increased pressure of gaseous reactants
increased surface area of solid reactants.(decrease particle size)
use of a suitable catalyst.
Anything that increases the number of successful collisions between reactant particles will speed up a reaction.
What factors affect the rate of reactions?
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Collisions and reactions: summary
© Boardworks Ltd 200716 of 39
© Boardworks Ltd 200717 of 39
Temperature and collisions
How does temperature affect the rate of particle collision?
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Effect of temperature on rate
The higher the temperature, the faster the rate of a reaction. In many reactions, a rise in temperature of 10 °C causes the rate of reaction to approximately double.
Why increasing the temperature will increase the rate of reaction?
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Temperature and particle collisions
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Effect of temperature on rate
Increasing the temperature will make the particles move faster, so there will be more collisions.
At a higher temperature, many more particles will have the necessary activation energy. The ratio of successful collision to unsuccessful collisions will increase.
Which one will contribute more towards increasing the rate of reaction?
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How does temperature affect rate?
The reaction between sodium thiosulfate and hydrochloric acid produces sulfur.
Sulfur is solid and so it turns the solution cloudy.
How can this fact be used to measure the effect of temperature on rate of reaction?
hydrochloricacid
sodiumchloride
sulfursodium
thiosulfate + + watersulfur
dioxide ++
Na2S2O3
(aq)2HCl(aq)
2NaCl(aq)
S(s)++
SO2
(g)H2O(l)
+ +
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The effect of temperature on rate
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Effect of concentration on rate of reaction
The higher the concentration of a dissolved reactant, the faster the rate of a reaction.
Why does increased concentration increase the rate of reaction?
At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react.
higher concentrationlower concentration
The ratio of successful collisions to unsuccessful collisions will stay the same, but there will be more successful collisions.
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Concentration and particle collisions
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The effect of concentration on rate
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Effect of pressure on rate of reaction
The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react.
Why does increasing the pressure of gaseous reactants increase the rate of reaction?
As the pressure increases, the space in which the gas particles are moving becomes smaller.
lower pressure higher pressure
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© Boardworks Ltd 200729 of 39
Effect of surface area on rate of reaction
Any reaction involving a solid can only take place at the surface of the solid.
If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction?
The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.
This means that there is an increased area for the reactant particles to collide with.
low surface area high surface area
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Surface area and particle collisions
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Reaction between a carbonate and acid
Marble chips are made of calcium carbonate. They react with hydrochloric acid to produce carbon dioxide.
The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes.
hydrochloricacid
calciumchloride
calciumcarbonate + + water + carbon
dioxide
CaCO3 (aq)
2HCl(aq)
CaCl2
(aq)++
H2O(aq)
+ CO2
(g)
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The effect of surface area on rate
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reaction (time)
ener
gy
(kJ)
What are catalysts?
Catalysts are substances that change the rate of a reaction without being used up in the reaction.
Catalysts never produce more product – they just produce the same amount more quickly.
Different catalysts work in different ways, but most provide an alternative path with lower activation energy (Ea).
Ea withcatalyst
Ea withoutcatalyst
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How do catalysts work?
More of the reactants will possess this lower activation energy.
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Everyday catalysts
Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils).
Many catalysts are transition metals or their compounds. For example:
Platinum is a catalyst in the catalytic converters of car exhausts. It catalyzes the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen.
Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).
© Boardworks Ltd 200736 of 39
© Boardworks Ltd 200737 of 39
Glossary
activation energy – The amount of energy needed to start a reaction.
catalyst – A substance that increases the rate of a chemical reaction without being used up.
concentration – The number of molecules of a substance in a given volume.
enzyme – A biological catalyst.
rate of reaction – The change in the concentration over a certain period of time.
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Anagrams
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Rates of reaction: summary
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Multiple-choice quiz